On July 23, 2003,University of Pittsburgh A multinational team of scientists led by Dr. Scranton announced that they used the "Wilkinson Microwave Anisotropy Probe" (WMAP for short) to observe cosmic microwaveBackground radiationAnd found direct evidence for the existence of dark energy.
The 840kg WMAP was launched on June 30, 2001. After the three-stage flight around the earth moon system, it was ejected to the second Lagrange point L2 of the sun earth system. This point is outside the lunar orbit, about 1.5 million kilometers away from the earth. Its surrounding area is the saddle point of gravity, where the satellite can maintain a similar distance from the earth, requiring little maintenance work,WMAP is maintained about four times a year.While the Earth Moon system rotates around the sun,WMAP is still spinning at 0.464 rpm and precessing at 1 rpm on L2 track.To reduce system error,WMAP accurately measures the temperature difference between any two directions in the sky, 180 degrees to 0.25 degrees apart.For all day information,WMAP adopts a complex all day scanning mode, which takes six months to complete a full day scanning.The first published data (2003) contains the results of two sets of all day scans.
application
All day anisotropy measured by WMAPdataTo be transmitted back to the earth, after complex data calibration and data processing, just like a map, we can use these data to draw a map about the radiation anisotropy of the whole day. Any point on the map records the temperature rise and fall in the corresponding sky direction.The map is drawn by Molleweide projection method, which maps the all day anisotropic information to a 2:1 ellipse, keeping the horizontal line as a straight line, and the meridian except the middle line isellipseArc, and keep the area enclosed by adjacent parallel lines and meridians unchanged.WMAP gives a total of five band full sky charts: W-band (~94GHz),V-band（～61GHz），Q-band（～41GHz），Ka band (～ 33GHz) and K-band (～ 23GHz).It is selected to reduce the prospectradiationThe ratio of CMB anisotropy to foreground radiation pollution is the largest at these frequencies.Among them,K-band and Ka band do not need to do CMB analysis, because they have the largest prospect of pollution and their observed areas in the l-space are limited by the uncertainty brought by other frequency band measurements.
In order to get the information of CMB anisotropy, it is important to understand the diffuse galactic radiation and extragalactic point light sources in order to remove these pollution information.By adopting Kp0Kp2 mask, linear combination of multi band WMAP data, removal of power and SZ effects and other means, finally get the anisotropic information of CMB.
brief introduction
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Wilkinson Microwave Anisotropy Detector
The Wilkinson Microwave Anisotropy Probe (WMAP) is a satellite of NASA, which aims to detect the residual radiant heat in the universe after the Big Bang. On June 30, 2001,WMAP carried Delta II rocket in Cape Canaveral, FloridaKennedy Space CenterLaunch.
The goal of WMAP is to find outCosmic microwave background radiationThe tiny difference between the temperatures helps test various theories about the creation of the universe.It is the successor of COBE and one of the intermediate explorer satellite series.
WMAP is named after David Wilkinson, a forerunner of cosmic background radiation.
WMAP operates at L2 point around the sun earth system, 1.5 million kilometers away from the earth.
Scientific purpose
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Wilkinson Microwave Anisotropy Detector
according toBig Bang Cosmic Model(The Big Bang), when the universe was about 300000 years old,universeInmaterialfromionizationState changes to neutralatomState of the universephotonComponents become microwave background radiation (CMB) due to decoupling with objects.For a given cosmic model,physical scientistWe can accurately calculate the CMB anisotropic power spectrum, which is related to the basic parameters of the universe model. Therefore, by accurately measuring the CMB power spectrum in a wide angle range, we can determine the basic parameters of various universe models, judge which universe models better describe our universe, and through these basic parameters,We can know many basic problems in cosmology, such as the geometry of space, the composition of matter in the universe, the formation of large-scale structures and the ionization history of the universe.CMB was first discovered by Penzias and Wilson (1965).In 1992,NASA's COBE satellite observation shows that CMB is a blackbody radiation spectrum that we can measure in nature, and it is the first time to give evidence of CMB anisotropy.However, due to the limitation of technology at that time,The angular resolution of COBE is only 7 degrees,The angular resolution of WMAP is 13 minutes, so WMAP will be able to accurately answer many basic questions above.
As the "ember" of the "Big Bang", the cosmic microwave background radiation was generated about 380000 years after the "Big Bang", of whichphotonTraveling through the universe will experience a series of physical processes, especially when passing through a galaxy with a large mass, these photons will encounter a "gravitational trap".The detection results show that the age of the universe is about 13.7 billion years, and the universe consists of 22.7% dark matter and 72.8% dark matterenergy, 4.5% of ordinary substances.The things that account for the largest proportion in the universe are the latest and most difficult for human beings to understand. So far, we only know that they exist, but we still do not know their nature.
Correlation measurement
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Discovery of Wilkinson Microwave Anisotropy Detector
The space orbit of WMAP (a) and the gravitational potential near L2 (b)
The Wilkinson Microwave Anisotropy Detector provides many more accurate values for the measurement of cosmological parameters than earlier instruments.According to the current universe model, it displays:
The age of the universe is 13.7 billion ± 200 million years old.
The cosmic mass energy group is:
4.5% general baryon matter, namely visible matter.
22.7% are dark matter of unknown type, which neither radiates nor absorbs light.
72.8% is mysterious dark energy, which causes the acceleration of the expansion of the universe.
Based on the data of the past three years, although there are still unexplained quadrupole moment anomalies in the measurement of large angles, the explanation of the universe expansion has been better improved.
The Hubble constant is 70 (km/s)/millisecond difference+2.4/- 3.2
The data show that the universe is flat.
The results of the polarization of the cosmic microwave background radiation provide an experimental demonstration that the cosmic expansion tends to be simplified in theory.
